Constraining West Antarctic snow accumulation and firn densification processes with GNSS reflectometry

Funding Program: NASA GNSS Research
Start Date: 1 September 2020
End Date: 31 August 2023
Collaborating Institutions: NASA Goddard Space Flight Center

Projections of ice sheet mass balance and consequent sea-level rise are complicated by large uncertainties in modeling surface mass balance processes, such as snow accumulation, snow redistribution, melt, and firn compaction. In situ observations for constraining model physics and validating coupled atmosphere-ice sheet models are sparse, especially in Antarctica where surface mass balance amounts to approximately 2600 Gt of mass input per year. With more than 7 mm/yr of sea-level equivalent accumulating as snow across Antarctica, surface mass balance variability has the potential to substantially impact the rate of sea-level rise, currently estimated to be ~3 mm/yr. We propose to use an existing array of 118 long-term global navigation satellite system (GNSS) instruments from the West Antarctic Ice Sheet and Ross Ice Shelf to observe net snow accumulation and firn compaction across highly variable surface mass balance regimes.

Classical GNSS processing for glaciological applications uses direct-signal positioning techniques to resolve three dimensional ice-sheet motion. Instead, we will use the reflected GNSS signal as a signal of opportunity to generate time series of the height of each GNSS antenna above the snow surface using interferometric reflectometry (GNSS-IR). The GNSS stations we will use were deployed for variable time periods, ranging from two weeks to over ten years, providing both short- and long-term insight into snow accumulation, melt, and firn compaction. Our project will consist of four interrelated objectives: (1) Optimization of GNSS-IR processing for higher-resolution surface evolution both in time and in space for understanding the scientific questions this method can answer; (2) Use GNSS-IR time series to validate radar-sounding- and satellite altimetry-derived snow accumulation and firn compaction rates across variable surface mass balance regimes; (3) Improve the representation of surface processes in firn densification models and optimize model calibration using GNSS-IR results; and (4) Compare long-term trends between GNSS-IR observations and firn densification models to quantify and partition uncertainty to its sources.

Our work will integrate GNSS-IR observations with ground-based terrestrial laser scanning, NASA’s Operation IceBridge radar sounding surveying, European Space Agency’s radar altimetry missions (ERS-1/-2, Envisat, CryoSat-2), and NASA’s ICESat-2 laser altimetry mission. We will interpret these observations in the context of state-ofthe-art climate models developed at NASA Goddard Space Flight Center (MERRA-2, NASA-GSFC firn model) in order to validate and improve the capabilities of these models. Our effort to observe surface mass balance processes with GNSS-IR will be particularly important for ground validation of ICESat-2 altimetry, for reducing uncertainty in the conversion of ICESat-2-observed ice-volume change to ice-mass change, and for pushing forward our ability to combine radar and laser satellite altimetry results. Finally, the proposed work will help drive the development of surface-process models in Antarctica that effectively capture spatial scales driven by topographic variability and time scales of single weather events, one of the goals deemed “Most Important” by the 2017 NASA Decadal Survey (H-1c), as well as contribute many other climate variability questions (e.g., C-1a, C-1c, C-8b) laid out by this report.